You often have to do a TON of reading and synthesis in H or SS fields. Mind-boggling reading lists aren't uncommon. If you're lucky the prose is a friendly style. If not... well, you know that feeling of trying to take in a math text quickly? It's not quite that bad, but it's related, lots of unfamiliar phrasing and terminology that you have to work out the connections between and condense semantics from. And then... you may not have well-defined problems. Essay questions/assignments amount to "come up with some thesis and supporting arguments that show familiarity with the material and the ability to generate interesting insight." Do "interesting" and "shows familiarity with the material" sound like uncomfortably subjective assessments? Why yes. Yes they are. Good luck! Also measuring things and experiments are hard. You can be reasonably dull in the natural sciences and still gather a useful amount of empirical observation; in the social sciences or humanities you're going to have to be pretty clever to get good empirical observations at all.

OTOH I would agree that CS and Math often have an additional level of conceptual difficulty, especially the more abstract the corner of the field you're working with, from "you'll definitely need some well-chosen concrete examples as introductory points and probably a bunch of graduated problem sets leading to the eventual brain-twisting revelation" to "you don't understand this, you just get used to it."

I'm considering a grad program that blends the two, so maybe I'll change my mind when I'm done.